A commercially important polymeric material is the polyamide known as nylon. A form of nylon (specifically, nylon-6) can be produced from caprolactam (CPL) utilizing the process described in FIG. 1. At a first step, CPL is mixed with water and initiators (such as, for example acetic acid). It is noted that the initiators are optional. It is also noted that other optional materials can be mixed with the CPL and water, including terminators and additives.
In a second step, a temperature of the mixture is increased to about 200.degree. C. under pressure, which causes the ring of CPL to open. The ring opening is shown below in reaction 1, and forms 6-aminohexanoic acid (6-AHA). EQU CPL+H.sub.2 O.revreaction.H.sub.2 N--(CH.sub.2).sub.5 --COOH (6-aminohexanoic acid, 6-AHA) (monomer, n=1) I
In a third step of the FIG. 1 process, the monomeric 6-AHA is heated to from about 240.degree. C. about 260.degree. C. to form polymers comprising 15 to 20 of the monomeric units. The reactions occurring during the initial polymerization comprise the addition reactions shown below as reactions II-IV, and condensation reactions shown below as reactions V and VI. The reactions referred to as "addition" (specifically, reactions II-VI) are identical in principle to the reactions referred to as "condensation" (specifically, reactions V and VI). However, an important distinction between the addition reactions and the condensation reactions is that condensation enables rapid chain growth with emission of only a single water molecule, whereas addition enables chain growth of only one monomeric unit per water molecule formed. This can enable condensation reactions to yield a greater rate of polymer growth than addition reactions. However, such is not always the case, as kinetics can become limited by the relative scarcity of available groups to perform polymerization as monomers become incorporated into polymeric units. EQU 2(6-AHA).revreaction.H.sub.2 N--(CH.sub.2).sub.5 --CO--NH--(CH.sub.2)5--COOH+H.sub.2 O (dimer, n=2) II. EQU dimer+6-AHA.revreaction.H.sub.2 N--((CH.sub.2).sub.5 --CO--NH).sub.2 --(CH.sub.2).sub.5 --COOH+H.sub.2 O (trimer, n=3) III. EQU trimer+6-AHA.revreaction.H.sub.2 N--((CH.sub.2).sub.5 --CO--NH).sub.3 --(CH.sub.2).sub.5 --COOH+H.sub.2 O (tetramer, n=4) IV. EQU pentamer+hexamer.revreaction.undecamer (n=11)+H.sub.2 O V. EQU undecamer+octamer.revreaction.nonadecamer (n=19)+H.sub.2 O VI.
In the above reactions I-VI, "n" equals the number of monomeric units in a chain. Generally, addition reactions produce chain links wherein "n" is from about 5 units to about 20 units, and then the reaction changes character so that condensation (sometimes referred to as polycondensation) begins to predominate. Reactions I-VI are all fully reversible, with equilibrium constants somewhat greater than 1 for the reactions as written.
It is noted that the above-described reaction I utilizes water as a reactant, and reactions II-VI generate water as a by-product. The presence of water in the reaction mixture allows the reversal of every step of the polymerization processes of reactions II-VI, and accordingly the reacting materials utilized in reactions II-VI are typically flushed with a gas during the reactions to remove water from the reacting materials. The gas typically is a material which is inert relative to reaction with the materials of reactions II-VI, and can comprise, for example, nitrogen or argon. Because water is necessary for reaction I, and yet causes reversal of reactions II-VI, water is allowed to escape from the reaction chamber as steam after reaction I. The reaction chamber may also be sparged with an inert gas during this time to aid in the steam's escape. Removal of water can be an important determinant on the rate and degree of polymerization occurring under particular reaction conditions. (Mallon and Ray, Journ. of Applied Polymer Science 69, 1203 (1998); and U.S. Pat. No. 5,269,980). The rates of two amidation reactions can, in practices frequently be determined by a rate of water removal under a variety of conditions, including melt conditions, solid state polymerization (SSP) conditions, or conditions wherein the reactions are in the form of small droplets (see, for example, U.S. Pat. No. 5,269,980). Mallon and Ray have shown that the rate of solid state polymerization of polyamides can be dependent on the diffusion rate of water generated by an amidation reaction, and specifically by the rate at which water diffuses to the surface of chips of solid nylon and escapes into a surrounding environment.
Steps 1-3 of FIG. 1 typically occur in a pressurized vessel. The steps are described as separate steps because they are chemically distinct stages of a reaction process. However, it is to be understood that the steps typically do not comprise separate manipulations of a reacting mixture, but rather comprise different stages along a reaction continuum.
Referring to step 4 of the FIG. 1 process, a pressure of a vessel comprising the reacting mixture is reduced and water is vented. Such venting and reduction of pressure can be accomplished by, for example, opening a valve of a pressurized reacting vessel to allow escape of water vapor and other gases from the vessel. The de-pressurized reacting mixture is maintained at a temperature of from about 240.degree. C. to about 260.degree. C. to encourage condensation within the mixture and form polymers comprising from about 50 to about 200 monomeric units. The material formed in step 4 is extruded, cooled and chopped to form nylon chips in processing labeled as step 5 of FIG. 1. It is noted that the material of step 4 typically comprises a thick, sticky substance, and such substance is typically extruded through a number of holes to form long strands. The strands are then cooled and chopped to form chips on the order of about 1/8 inch in length and about 1/8 inch in diameter.
Referring to step 6 of FIG. 1, the chips are leached in hot water in a multiple-step process that takes from 15 to 20 hours to leach unreacted CPL from within the chips. The chips typically comprise about 10% unreacted CPL, as a result of the reactions I-IV being in equilibrium, and the leaching enables such CPL to be reclaimed from the chips. The leaching typically occurs at from 105.degree. C. to 120.degree. C.
Referring to step 7, the chips from step 6 of FIG. 1 are dried. Such drying typically comprises exposing the chips to warm nitrogen in a tumble dryer under a partial vacuum in a process that typically takes a number of hours.
After the chips are dried, they can be utilized in one or more of the FIG. 1 steps 8, 9 and 10; which comprise subjecting the chips to a solid state polymerization (SSP), selling the chips, and using the chips to form nylon products, respectively.
A quality of nylon chips is measured by a so-called formic acid viscosity (FAV) value, a measure of relative viscosity that reflects an average chain length within the nylon material. The formic acid viscosity value is determined with an 8.4% (weight/weight) solution of the nylon material in 88% formic acid, and reflects a viscosity of such solution. Higher FAV values typically indicate longer chain lengths within the nylon chips, and correspond to chips having higher value than would chips with lower FAV values.
The FAV values of chips at the processing of step 7 of FIG. 1 are typically about 40. If the chips are subjected to the SSP of step 8, the FAV values can be increased to 200 or higher. Nylon-66 undergoes SSP more rapidly and can reach FAV values of 500 or more. FAV values greater than 200 could, in theory, be obtained from the nylon-6if the nylon were held in the reaction chamber of step 3 for a longer period of time. However, such is typically not practical because the increased viscosity of the nylon material would significantly slow a reactor's capacity, as well as complicate extrugion, pelletizing and leaching. Additionally, as the polymer melt becomes more viscous, difficulties in heat transfer can occur. Specifically, as the walls of a reactor become coated with highly viscous nylon material, it becomes difficult to stir the material within the reactor which can lead to non-homogeneous heat transfer, and eventually to charring of material within the reactor. In light of these complications, SSP is a preferred method of increasing the viscosity of nylon chips.
In a common method for accomplishing SSP, the dried chips from step 7 of FIG. 1 are placed in a reactor and subjected to heat and vacuum, or heat and an inert gas stream to remove product water, to cause the polymerization shown in reaction VII (below), wherein "n" and "m" are integers. EQU H.sub.2 N--((CH.sub.2).sub.5 --CO--NH).sub.n --(CH.sub.2).sub.5 --COOH+H.sub.2 N--((CH.sub.2).sub.5 --CO--NH).sub.m --(CH.sub.2).sub.5 --COOH.revreaction.H.sub.2 N--((CH.sub.2).sub.5 --CO--NH).sub.(n+m) --(CH.sub.2).sub.5 --COOH VII.
A given sample of nylon tends to consume molecules with smaller values of "n" and "m" first during an SSP reaction cycle because the mobility of the nylon molecules and the relative proportion of free reactive ends decreases as a function of molecular weight. In principle, the SSP reaction could proceed until the entire batch consisted of a single very large nylon-6 molecule. However, in practice such does not occur; instead the SSP reaction is stopped when a viscosity reaches a desired value. Although reaction VII is shown relative to condensation of nylon-6, it is to be understood that similar condensation reactions occur for other polymers.
Typically, the highest value of viscosity reached by nylon-6 corresponds to an FAV value of 200. Proceeding beyond the FAV value of 200 is typically impractical due to the long reaction times required. Even reaching an FAV value of 200 with current technology requires reaction times on the order of 48 hours. The long reaction times result from decreasing numbers of free reactive ends, and the fact that in the solid state a significant portion of the otherwise free ends become entrapped in the crystalline portion of the solid, further slowing reaction rates. The crystal-trapped ends can in theory be liberated by remelting the mass, re-extrusion into strands, repelletizing and redrawing. In practice, however, this is generally not done due to increased costs and complexity.
It would be desirable to develop improved methods for forming polyamide materials, such as, for example, nylon-6. It would further be desirable to develop improved methods for accelerating SSP processes.